1. Field of the Invention
The present invention pertains to a passive membrane system which may be used to recover carbon dioxide from a carbon dioxide plant vent gas stream.
2. Description of the Background
Depending upon whether a gas mixture contains carbon dioxide (CO.sub.2) in a high or low amount, a variety of techniques have been used to separate CO.sub.2 from the mixture. For example, when the amount of CO.sub.2 present in the gas mixture is low, and purification cannot be achieved directly by cooling and partial condensation, it is possibly to scrub the gas mixture with a suitable solvent to dissolve the CO.sub.2, and then to strip the CO.sub.2 from the solution so obtained. The carbon dioxide obtained can then be compressed, dried, cooled and further purified by partial condensation or distillation.
When the gas mixture contains a high amount of CO.sub.2, however, the gas mixture may be compressed and then dried by absorption or other means. Finally, after removing undesirable impurities such as sulfur containing compounds, the mixture is cooled and after distillation CO.sub.2 is obtained as a bottoms product. Unfortunately, the overhead product of the distillation column will always contain a significant amount of CO.sub.2 which is inevitably wasted. This problem is particularly acute in conventional CO.sub.2 liquifaction plants.
In a typical CO.sub.2 liquifaction plant, a significant percentage of the CO.sub.2 feed is lost as stripper vent gas. For example, in a 200 ton/day CO.sub.2 liquifaction plant, about 10 to 15% of the CO.sub.2 feed is lost. The composition of this dry waste gas steam is usually 75% or more of CO.sub.2 with the remainder being N.sub.2, O.sub.2, H.sub.2, and CH.sub.4 with trace amounts of NH.sub.3, CO and sulfur containing compounds.
In order to address this problem, at present, carbon dioxide is recovered from gas mixtures by subjecting the gas mixture to membrane separation, recovering from the membrane separation a permeate having a carbon dioxide concentration between the equilibrium concentration and about 98% by volume and then distilling the permeate at subambient temperature above the freezing temperature of the permeate and recovering carbon dioxide as a liquid bottoms product of the distillation.
The above conventional process is described in U.S. Pat. No. 4,639,257. In this process, a distillation step is required and the only energy savings available is attained by recycling the overhead stream from the distillation step to the membrane separation step.
The process of U.S. Pat. No. 4,639,257 accomplishes the recovery of carbon dioxide by first directing a gas mixture through a membrane and thereafter distilling the permeate in a cryogenic unit. The process is modified depending upon whether the carbon dioxide is present in the gas mixture in a high concentration or in a concentration which is not greater than the equilibrium concentration at the freezing temperature of the mixture.
When carbon dioxide is present in the lesser amount, the gas mixture is subjected to membrane separation and is then distilled at sub-ambient temperature to recover substantially pure carbon dioxide as a liquid bottoms product of the distillation. In some cases, the overhead stream from the distillation is recycled to the membrane as noted above.
When carbon dioxide is present in a high concentration, however, the gas mixture is distilled at sub-ambient temperature in a distillation column, and substantially pure carbon dioxide is recovered as a liquid bottoms product of the distillation. The overhead stream from the distillation is warmed to approximately ambient temperature, and is then directed over a membrane, and the resulting carbon-dioxide rich permeate is recycled to the distillation column.
Notably, in the above process, the warming step is an essential step in the process as this patent teaches that the tail gas from the distillation must be warmed before contact with the membrane separation unit.
Thus, such a separation technique requires an external energy source for operation. This renders the process economics very unfavorable.
U.S. Pat. No. 4,595,405 discloses a process for the production of nitrogen using a cryogenic separation unit and one or more membranes, however, cryogenic separation units in combination with membranes were known prior to U.S. Pat. No. 4,595,405. See, for example, U.S. Pat. No. 4,180,553 and 4,181,675.
U.S. Pat. No. 4,595,405 discloses a process wherein air is fed to a cryogenic separation unit, and a portion of the output from the unit is then fed to a membrane unit to form a nitrogen-rich gas stream. Thereafter, the nitrogen-rich stream is returned to the cryogenic unit, and a purified nitrogen-rich stream is recovered therefrom.
This patent discloses that the use of the membrane provides "an additional degree of freedom" to adjust the process parameters for further optimization. This only appears to mean, however, that the pressure of the various streams no longer must be determined by ambient pressure but may be set at substantially any desired level. However, this patent is clearly not specifically addressed to the separation of carbon dioxide from vent gas mixtures. Further, the disclosed process does not use the existing temperatures and pressures of a carbon dioxide plant in separating carbon dioxide from vent gas mixtures.
Thus, it would be extremely desirable if a method for separating CO.sub.2 from a gas mixture could be effected without the use of an external energy source for a warming step and in an economically advantageous manner using the temperature and pressure of the CO.sub.2 plant It would also be extremely desirable if vent gas which is lost from conventional CO.sub.2 liquifaction plants could be treated to recover CO.sub.2 in a completely passive manner using the pressure of the waste gas stream.